The use of split retaining rings on shafts is wellknown. Basically, a split ring consists of a circle of spring steel or the like having a split at one location on the circumference of the ring. The split allows the ring to be expanded to fit over a shaft, but the natural spring resilience of the ring allows it to snap into an accommodating groove cut into the shaft and thus the ring may serve as a retaining means to either fix the shaft in position or retain other elements on the shaft. In the course of mounting and demounting split retaining rings on a shaft by means of a hand tool, it would be desirable to have a tool requiring a minimum of adjustment and which would permit the ring to be expanded yet avoid a mechanism that could expand the ring beyond its elastic limit.